13.4.2—Stereoisomerism and its division

Stereoisomers have the same atoms joined in the same order but differ in spatial arrangement. Geometrical isomerism occurs when rotation is restricted, while optical isomerism arises from a chiral centre and produces non-superimposable mirror images.

A C=C can give cis/trans forms only when each carbon has two different groups. A tetrahedral carbon is a chiral centre when it is bonded to four different groups.

cis-but-2-ene and trans-but-2-ene are geometrical stereoisomers. A molecule with one carbon attached to H, OH, CH₃ and CH₂CH₃ has an optical stereocentre.

Same molecular formula alone is insufficient. Check connectivity first, then test the structural condition for cis/trans or chirality.